dialect/sql/sqljson: support comparing values with null literal

dialect/sql/sqljson/sqljson.go

@@ -26,6 +26,30 @@ func HasKey(column string, opts ...Option) *sql.Predicate {
 	})
 }
 
+// ValueIsNull return a predicate for checking that a JSON value
+// (returned by the path) is a null literal (JSON "null").
+//
+// In order to check if the column is NULL (database NULL), or if
+// the JSON key exists, use sql.IsNull or sqljson.HasKey.
+//
+//	sqljson.ValueIsNull("a", sqljson.Path("b"))
+//
+func ValueIsNull(column string, opts ...Option) *sql.Predicate {
+	return nullP(sql.OpEQ, column, opts...)
+}
+
+// ValueNotNull return a predicate for checking that a JSON value
+// (returned by the path) is not a null literal (JSON "null").
+//
+// In order to check if the column is not NULL (database NULL), or
+// if the JSON key exists, use sql.NotNull or sqljson.HasKey.
+//
+//	sqljson.ValueNotNull("a", sqljson.Path("b"))
+//
+func ValueNotNull(column string, opts ...Option) *sql.Predicate {
+	return nullP(sql.OpNEQ, column, opts...)
+}
+
 // ValueEQ return a predicate for checking that a JSON value
 // (returned by the path) is equal to the given argument.
 //
@@ -424,6 +448,28 @@ func ParsePath(dotpath string) ([]string, error) {
 	return path, nil
 }
 
+// nullP creates a predicate that compares (using the given operator)
+// the JSON value to the JSON null literal.
+func nullP(op sql.Op, column string, opts ...Option) *sql.Predicate {
+	return sql.P(func(b *sql.Builder) {
+		switch b.Dialect() {
+		case dialect.MySQL:
+			ValuePath(b, column, opts...)
+			b.WriteOp(op).WriteString("CAST('null' AS JSON)")
+		case dialect.Postgres:
+			ValuePath(b, column, append(opts, Cast("jsonb"))...)
+			b.WriteOp(op).WriteString("'null'::jsonb")
+		case dialect.SQLite:
+			path := &PathOptions{Ident: column}
+			for i := range opts {
+				opts[i](path)
+			}
+			path.mysqlFunc("JSON_TYPE", b)
+			b.WriteOp(op).WriteString("'null'")
+		}
+	})
+}
+
 // normalizePG adds cast option to the JSON path is the argument type is
 // not string, in order to avoid "missing type casts" error in Postgres.
 func normalizePG(b *sql.Builder, arg interface{}, opts []Option) ([]Option, interface{}) {